An airplane's airframe and engines produce varying amounts of objectionable audible noise during different flight conditions. During departure, the engines produce most of the noise; however, during approach, airframe noise is a much greater factor. Airframe noise is generated by air flowing over the airplane's fuselage, landing gear, wing leading and trailing edges equipped with high-lift devices and flap systems. On the approach to landing, high lift systems, such as wing leading edge devices and wing trailing edge flap systems, are extended and the engines are operated at low thrust levels. Accordingly, the most audible noise produced by flap systems occurs on approach when the flaps are extended and lateral flap edges are exposed. Furthermore, recent advances in technology have reduced engine noise significantly during all flight conditions. Therefore, airframe noise has become a more dominant source of aerosound, and particularly so for an airplane during approach to landing.
One of the main noise-generating mechanisms at play in flap edge flows can be attributed to shear layer fluctuations and their interaction with surrounding edges and surfaces. When an airfoil creates lift, the inherent pressure differences between the top and bottom surfaces cause air to leak around the edges of the airfoil in an attempt to equalize the pressure. Since flap edges are usually sharp, this results in a separated shear layer emanating from the bottom side of the airfoil. Similarly another vortex emerges from the sharp junction between the side edge 108 and the upper surface of the flap 110. As shown in FIG. 1, this occurs throughout the chordlength 102, and the shear layer rolls up (reference number 104 represents this roll up effect) into a vortex core 106 which is transported downstream. These two vortex systems dither around the surrounding surfaces, emitting broadband noise. At some angle-of-attack dependent chord-wise location, the lower vortex core will spill over the sharp top surface edge 108 creating additional broadband noise.
After spilling over the sharp edge 108, the lower surface vortex core interacts with the upper surface vortex. As shown in FIG. 2, the two vortex-systems then entrain each other and develop into a high-intensity tightly formed vortex core 214 which can be detrimental for airplanes crossing the wake further downstream
In the past, various approaches have been taken to attenuate the vortices produced by flaps and other lifting surfaces. Flap edge fences have been proposed as a way to mitigate the noise source emerging from the flap edges. As will be explained in more detail below, a general flap edge fence (not shown in FIG. 2) forms a barrier that modifies and displaces the flap tip vortex system 214 leaving the flap edge 208.
One of the problems facing the successful installation of such a flap edge fence is that, while highly beneficial with a fully extended flap trailing edge, the fences are far from optimal in high-speed cruise conditions. Mounted perpendicularly to the deployed flap trailing edge, the fence may form a significant angle to the local air-flow when the flap trailing edge is stowed due to Fowler motion and inherent cross flow of any three-dimensional wing. This would result in wasted lifting forces and additional drag, which in turn creates unnecessary loads on the structure. Any such additional drag in cruise should be kept to a minimum for efficient operation. Currently, existing flap edge fences are fixed and no alternatives are available.
To minimize aerosound due to the extension of flaps during aircraft operations and, more specifically, to reduce drag during high-speed cruise conditions, it is desirable to have a deployable flap edge fence so that any additional drag in cruise is kept to a minimum for efficient operation. Other desirable features and characteristics of embodiments of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.